1. Field
The current disclosure relates to a method for forming, in a semiconductor substrate or the like, features disposed at a pitch shorter than a minimum printable pitch of a lithography process.
2. Description of Related Art
Photo-lithography (referred hereinafter generally as “lithography process”) is an enabling technology in semiconductor device manufacturing. There are many market factors as well as many technological factors which continually drive semiconductor manufacturers to fabricate devices having features with smaller dimensions from one generation to the next. As such, there are significant efforts undertaken by semiconductor device manufacturers to develop processes that may be used to produce features that are smaller in size than a minimum printable dimension (MPD) of a lithography process.
The MPD of a lithography process is the smallest dimension that can be resolved in a photoresist layer given the focus, light energy, wavelength, and other parameters of the light source used in the lithography process. In forming hole patterns, for example, the minimum printable dimension of an Argon Fluoride (ArF, with wavelength λ=193 nm) immersion lithography process is nominally 50 nm. As such, for example, circular features with diameter substantially shorter than 50 nm cannot be resolved in a photoresist layer utilizing simply an ArF immersion lithography process. While features with diameter substantially shorter than 50 nm can be achieved with an Extreme Ultraviolet (UV) (λ=13.5 nm) lithography process, unlike the aforementioned ArF immersion lithography process EUV is not suited for high volume manufacturing.
One conventional technique that may be used to reduce the size of a feature below the MPD of a lithography process, like the ArF immersion lithography process, is the use of a directed self-assembly (DSA) layer following a first step of patterning a photoresist layer. A DSA layer is formed by a polymer-blend DSA comprising two distinct polymeric species, hydrophilic polymer and hydrophobic polymer.
In a polymer-blend DSA, one of the two polymeric species has an affinity for the photoresist layer whereas the other species does not. This differential affinity to photoresist may be advantageously used to trigger a self-assembly process that confines each polymeric species to desired regions. In the context of DSA, the self-assembly process is typically activated using a thermal treatment (or cycle) of the DSA layer.
Upon subjecting the DSA layer to a thermal cycle, the polymeric species that has poor affinity to the photoresist assembles away from regions where there is photoresist whereas the polymeric species that has strong affinity for the photoresist layer assembles in proximity to the photoresist. Following the thermal cycling, the polymeric species having poor affinity to the photoresist is removed by either a wet etching or a dry etching process; in either case, the etchant used has high selectivity against the species that have strong affinity to the photoresist, thereby allowing the selective removal of the species having poor affinity to the photoresist.
FIG. 1A-FIG. 1F show an example of an arbitrary lithography process with an added DSA process step for the purpose of obtaining circular features with diameter smaller than a MPD 104 of the arbitrary lithography process. FIG. 1A shows a top view of a semiconductor wafer 100 having disposed thereon a photoresist layer 102. The cross-sectional view of semiconductor wafer 100 along a symmetry line R-R′ is shown in FIG. 1B. The substrate 101 is shown in a cross-sectional view of the semiconductor wafer 100. Although, for ease of description, substrate 101 is assumed to be a semiconductor material, an artisan of ordinary skill in the art will readily understand that photoresist layer 102 may also be applied to the top layer of a stack of layers disposed on substrate 101; such a stack of layers may be, for example, a stack of dielectric layers. Alternatively, the photoresist layer 102 may also be applied to a single layer disposed on substrate 101.
As shown in FIG. 1A, a pattern is formed within photoresist layer 102. (Steps of coating, exposing, and developing photoresist layer 102 to form the pattern are omitted for clarity.) Features 103a, 103b, and 103c are holes aligned along the symmetry line R-R′, and they are circles with diameter equal to the MPD 104 of the arbitrary lithography process. Further, features 103 are disposed at a minimum printable pitch (MPP) 105 of the arbitrary lithography process (the MPP is described later below).
FIG. 1C shows a top view of semiconductor wafer 100 having a patterned photoresist layer 102 coated with a DSA layer 106, after a thermal cycle which activates the self-assembly process, and after the selective etching of the polymeric species having poor affinity for photoresist layer 102. This DSA process step effectively forms features 107a, 107b, and 107c which are concentric with features 103a, 103b, and 103c respectively. The corresponding cross-sectional view of FIG. 1C along the R-R′ line is shown in FIG. 1D. Although not shown in FIG. 1D, the DSA layer 106 may be formed at a bottom of a hole formed by the features 103 and may remain at the bottom of the hole formed by the features 103.
The pattern formed in DSA layer 106 can then be transferred into substrate 101 via either a wet or a dry etching process wherein the etchant has higher selectivity to the substrate 101 than to the DSA layer 106. The resulting structure from the etching process (after stripping the substrate of DSA layer 106 and underlying photoresist layer 102) is shown in FIG. 1E (top view) and FIG. 1F (cross-sectional view along R-R′). As can be seen in FIG. 1E, the features 107a, 107b, and 107c each have a diameter smaller than MPD 104.
While it is possible, as described above, to obtain features smaller in size than MPD 104 utilizing a conventional lithography process with an added conventional DSA step, it is not known how to use these two techniques to produce features that are disposed at a pitch shorter than the minimum printable pitch of the lithography process using at most one photoresist patterning step. The minimum printable pitch (MPP) may be described as the distance measured from center to center of two adjacent features, each having a dimension equal to the MPD. As shown in FIG. 1E, while features 107 are smaller in diameter than MPD 104, features 107 are still disposed at MPP 105.